The present invention relates to a warning or alarm system for protecting a structure against intrusion by breaking or cutting through, or climbing over partitions, or walls around or within a structure. More particularly, the present invention relates to such a system wherein a vibration sensitive detector, particularly a shielded electrical cable which produces an electrical signal due to the minute flexing of the cable caused by vibrations, is attached to the partition, and the vibrations produced when an attempted intrusion occurs are detected and processed to provide an alarm to indicate the attempted intrusion and particularly whether a cut or break-through (short term event) type intrusion or a climb over (sustained activity) type of intrusion is being attempted. A system of the above general type is disclosed in commonly assigned U.S. Pat. No. 4,365,239, issued Dec. 11th, 1982 to R. Mongeon.
When an insulated electrical cable is flexed, or when pressure is applied thereto, the resulting stress produced in the previously uncharged dielectric material of the cable by the movement results, due to the triboelectric effect, in the generation of a very small electric signal which may be sensed with appropriate sensing circuitry. When such a cable is attached to a partition, or a fence, minute flexing of the cable due to vibration of the partition results in the generation of an electric signal corresponding to these vibrations. However, electric signals will be produced both by vibrations, and hence electrical cable vibrations, which are desired to be detected, i.e., vibrations caused by attempted intrusions, as well as by vibrations which are not desired to be detected, i.e. vibrations from extraneous sources, such as air conditioners, nearby carts, or normal activities in adjacent rooms or areas, etc. Since the detection of these extraneous source caused signals could lead to false alarms, special signal processing is required in order to distinguish signals originating from intrusion related vibrations from signals originating from extraneous source related vibrations.
In order to provide adequate protection for a wall or other partition, it is desired to know whether an intruder is attempting to break or cut through the partition or is attempting to climb over same. In general, the signals resulting from attempts to break, or cut through a partition are of short duration, are abrupt, and are generally repeated a number of times within a predetermined short period of time. On the other hand, signals corresponding to attempts to climb a partition, drill through it, or pressure a collapse, generally have longer duration and a lower base frequency-than cut-through types of vibration signals and persist for a longer period of time.
Research has determined that the different materials comprising a partition vary considerably in the frequency of the vibrations induced onto the vibration detector, i.e., the cable. Moreover, as indicated above, the frequency of the vibrations for a given partition may differ greatly and over a relatively wide frequency band depending on the type of intrusion. In view of the low level of the input signals, this can result in difficulty in differentiating the vibrations from background noise, particularly if a partition is generally vibrating at a given frequency for a known and non-intrusive source, resulting either in non-detection of the actual intrusion induced input signal or a false alarm due to an extraneous vibration source. To overcome this detection problem it is often necessary to, in effect, customize a particular system at the factory for a particular type partition or installation after taking measurements with special equipment at the site of the system installation to determine an optimum operating band width. Such, of course, is rather expensive and time consuming.